As IGF-1 tends to decrease with age, higher levels in young adults may be a youth marker.52,53 Reduced IGF-2 has been associated with aging in various organs and primordial germ cells while compromising mitochondrial functionality.54,55 In addition, upregulated FGF-21 and GDF-15 have been regarded as markers of mitochondrial stress and possibly dysfunction.56 Additional discussions on cytokines related to obesity, along with profiles of adipokines, myokines, and growth factors as signatures of molecular aging, can be found in eAppendix 4 and eTable 5 in Supplement 1. Here, FGF21 is linked to obesity due to melanocortin 4 receptor deficiency.